This invention relates to data communication over a multiplexed channel.
SONET/SDH (Synchronous Optical Network/Synchronous Digital Hierarchy) standards were developed as an evolution of legacy copper based transmission equipment to serve as a next generation/broadband transport of voice traffic over fiber optic infrastructure.
The first generation of digital transmission equipment used physical layer technologies that were encompassed under three regional digital signal hierarchies. The North American hierarchy consists of DS0 (64 kb/s), DS1 (1.544 Mb/s), DS1c (3.152 Mb/s), DS2 (6.312 Mb/s), DS3 (44.736 Mb/s), DS3C (91.035 Mb/s) and DS4 (274.176 Mb/s) signals. The European hierarchy consists of E0 (64 kb/s), E1 (2.048 Mb/s), E2, E3 and E4 signals. The Japanese hierarchy is similar to the North American hierarchy with differences at the higher speeds. All of the rates and formats listed above are multiples of the basic digitized voice signal (4 kHz audio bandwidth sampled at the Nyquist rate of 8000 samples per second and encoded into an 8-bit PCM signal). In years past, the primary purpose of the digital transmission equipment was to carry digitized voice traffic multiplexed into signals at various rates defined in the hierarchies outlined above.
At the higher of the of signal rates, broadband optical fiber links are used for communication at the physical layer. The majority of this broadband optical fiber communications are based today on the SONET/SDH family of standards (SDH is essentially the international standard corresponding to SONET). The standards provide mechanisms to transport digitized voice traffic streams within higher speed SONET “pipes,” which are aggregated streams of multiplexed low speed traffic. A series of Bellcore and ANSI specifications define data formats of payload containers (typically referred to as virtual tributaries, or VTs) to carry legacy traffic rates (DS1, DS1C, DS2 and DS3, of what is known as the PDH, or the Pleisochronous Digital Hierarchy) in higher speed synchronous communication on the optical links. According to the virtual tributary approach, data and voice traffic, regardless of their rates, are mapped to the strict hierarchy of virtual tributaries, which can be successively multiplexed into more aggregated streams.
Referring to FIG. 1, communication according to the SONET standard makes use of a ring architecture in which a number of communication nodes are connected by optical links to form a ring. A SONET ring 110 typically has a number of nodes each of which includes an add/drop multiplexer (ADM) 120. Each of the nodes are coupled to two neighboring node by optical paths 122. Communication passes around the ring in a series of synchronous fixed-length data frames. Each ADM 120 is configured at the time the ring is provisioned to pass a portion of the communication on the ring without modifying it, to extract (drop) a portion of the communication destined for that node, and to add outbound communication leaving the node to the optical path. The dropped and added communication passes between the ADM and local communication equipment, such as a multiplexer 130, which multiplexes a number of separate traffic streams 134. For example, the a separate communication streams 134 may be 1.5 Mb/s (DS1) data stream on which separate 64 kb/s (DS0) telephone channels that are multiplexed. The DS1 data stream is multiplexed onto the optical path and passed between particular nodes on the SONET ring. Typically, SONET ring, 110 is provisioned to provide fixed rate bidirectional communication streams, also known as virtual paths, between pairs of communication steams 134, each of which is coupled to a different ADM 120 on the ring. In operation, the virtual paths coupling different communication streams 134, including their allocated data rates, typically remains fixed for long periods of time.
A variety of types of SONET rings are used. In a UPSR (unidirectional path switched ring), all communication between nodes travels in one direction around the ring. In another type of ring, a BLSR (bi-directional line switched ring), communication travels between nodes on the ring in both directions, with two unidirectional optical links joining each pair of neighboring nodes.
The process of multiplexing standard rate data streams into higher rate streams is a basic feature of SONET communication. Multiplexed data streams passes between nodes in a SONET ring at particular data rates. Referring to FIG. 2, a hierarchy of standard rate streams and multiplexing of streams that are defined as part of the SONET standards is illustrated. At the lowest rates, a VT1.5 virtual tributary 220 supports a 1.5 Mb/s data rate. This is the data rate of a common DS1 (T1) service, and can support up to 24 separate 64 kb/s (DS0) data streams. A VT2 virtual tributary 222 supports 2 Mb/s data, and a VT6 virtual tributary 224 supports 6 Mb/s. These virtual tributaries are typically the smallest units of communication that are added or dropped at an ADM. Virtual tributaries can be combined into a: victual tributary (VT) group 230, which can consist of 4 VT1.5, 2 VT2 or 1 VT6 virtual tributaries, and entire VT groups can be added and dropped at an ADM.
In different configurations of SONET rings, communication on the optical links can be at different data rates and use various forms of multiplexing. In one mode, a series of synchronous frames each include a Synchronous Payload Envelop (SPE) 240, which can be used to carry 45 Mb/s data between the SONET nodes. The SPE can carry a raw data rate of 45 Mb/s or can be used to carry seven VT groups. An SPE is in turn; carried in an STS-1 frame 250, which adds control and overhead data to the SPE for transmission. The STS-1 frame can be optically encoded as an OC-1 signal 280 for transmission over an optical link, or multiplexed three STS-1 frames to an STS-3 frame 260 and optically encoded as an OC-3 signal for transmission over a higher capacity optical link. An STS-3 frame can also carry a concatenated STS3c payload envelope 252, which is used to carry a 140 Mb/s signal. The STS-3 frame can multiplexed still further, for instance four STS-3 frames to a STS-12 frame 270, which is in turn optically encoded as an OC-12 signal 284. Likewise, a SONET frame could be a concatenated STS-48c frame encoded as an OC48 optical signal, wherein a single SPE accounts for the entire OC48 payload.
Referring to FIG. 3, in the case of STS-1 framing, each of a sequence of STS-1 frames 310 carries an SPE 330. Each STS-1 frame is typically conceptualized as an array of 90 columns by 9 rows of 8-bit bytes. As a signal, the rows are concatenated one after another to form a linear signal. Each SPE 330 holds 87 columns by 9 rows, with the remaining 3 columns of the STS-1 frame being used for control and overhead data. In transmission, an SPE is not necessarily aligned with an STS-1 frame. Each STS-1 same has a line and section overhead section 320 that includes a pointer to the first byte of the SPE.
Each SPE 330 is 87 columns by 9 rows in size. The first column of the SPE is an STS path overhead 340. When the SPE holds seven VT groups 350, the VT groups each use 12 columns; 2 extra columns are left unused to fill the 87 column SPE. If the content of the SPE is not a set of VT groups, then the fill remaining 86 columns can be used for data.
As outlined above, each VT group 350 can multiplex one or more equal sized virtual tributaries. In FIG. 3, a VT group holding four VT1.5 virtual tributaries 360 is shown. The first byte of each virtual tributary 370 is used for control information for that virtual tributary. The content of the first byte four consecutive frames of a virtual tributary is assembled into a four-byte control quantity which is associated with the virtual tributary.
An explosion of Internet traffic and an ever increasing need to seamlessly network geographically disjoint locations has resulted in a dramatic increase in data traffic the past few years. Today, this data traffic is largely transported using the virtual tributaries of the SONET infrastructure. The volume of this data traffic now typically exceeds the volume of voice traffic.
The LAN backbone and access technologies that are used to pass data to and from the SONET infrastructure generally support different data rates than those of the PDH digital signal hierarchy described above. The technologies in use today and those emerging for the future include Ethernet (10 Mb/s), Fast Ethernet (100 Mb/s), Gigabit Ethernet (1 Gb/s), Frame Relay, ATM, DSL and Cable. The variety of rates and formats of data streams that are mapped to legacy Virtual Tributary rates can result in gross inefficiencies of “stranded” bandwidth that is left unused in a SONET Frame. In addition, the existing synchronous digital signal hierarchy was designed for static allocations of data rates over data streams between nodes and hence is not particularly flexible to deal with the bursty nature of data traffic. For example, a SONET ring may be provisioned according to the maximum data rates guaranteed between particular nodes, making it difficult to make use of unused capacity between one pair of nodes for data communication among another pair.
Some approaches to the data transport problems are based on the predominant networking protocols that are used to carry data on networks today, namely IP (Internet Protocol) and ATM (Asynchronous Transfer Mode). In particular, two types of approaches to passing data over SONET infrastructure have been proposed. A common theme in the two solutions, which are outlined below, is the retention of the ring topology of interconnection of the SONET ring, and support for redundancy and protection switching in the infrastructure.
A first approach, ATM Virtual Path (VP) Ring, is based on the Bellcore standard GR-2837-CORE. According to this standard, ATM is used as the protocol of choice to multiplex both data and voice streams in a SONET ring environment. ATM functionality is integrated into SONET transport equipment at each node. All traffic streams then pass over the SONET ring are adapted to ATM fixed length cells. The ATM cell stream is treated as a contiguous byte stream, which is treated as payload in all of a SONET SPE. ATM Virtual Paths are provisioned between various nodes on the ring to terminate different traffic streams. That is, particular data rates are allocated for communication between particular pairs of nodes in the ring. A typical ATM-based ADM has 2 logical layers: a SONET framer/demultiplexer which delivers an STS-N payload envelope, and an ATM VP multiplexer/demultiplexer which extracts appropriate Virtual Paths from the SONET payload envelope. In order to support IP-based communication over the ATM-based infrastructure, associated technologies such as LANE, MPOA, and MPLS are used.
To preserve the fixed rate, time division multiplexed (TDM) nature of SONET to transport voice, GR-2837-CORE also specifies a hybrid solution. In the hybrid approach, one or more STS-1 channels in a STS-N ring are reserved for ATM/VP Ring, and the remaining STS-1 channels used to carry traditional TDM traffic.
A second approach to passing data over SONET infrastructure is called Dynamic Packet Transfer (DPT). This approach uses IP as the basic multiplexing technology instead of ATM. Like the ATM VP Ring approach, the DTP approach assumes replacement of the existing transport infrastructure at each SONET node with IP-based routers. DPT defines a layer 2 protocol called Spatial Reuse Protocol (SRP), which is used to transport IP datagrams between nodes in a ring. SRP allows reuse of ring bandwidth across different sections of a ring of nodes. A supplementary algorithm called SRP Fairness algorithm (SRP-fa) was also defined to achieve fair sharing of bandwidth, between various nodes in a ring topology.